As is well known, circuits of the above-mentioned type have their field of application in a variety of devices for controlling the electric current flowing through a load, such as regulators and limiters of the current supplied to a stepper electric motor or a generic inductive load, hereinafter denoted by an "L".
In the instance of an electronically controlled stepper motor, for example, there exists a need to independently power the respective phases of the motor by having each of them connected to a corresponding control stage which may comprise a MOS-type or bipolar transistor bridge.
Each motor phase may be viewed in the main as an inductor wherein the current would tend to increase indefinitely unless it is regulated through the bridge circuit.
Heretofore, the control phase has been implemented in the so-called chopper mode, i.e. by activating and de-activating the control stage associated with each phase of the motor based on the electric current value sensed on the inductive load by a sensing circuit.
Thus, the current flowing through the inductive load has a sawtooth form due to that, as the sensing circuit senses a current value equal to a predetermined reference value, the driver stage is de-activated to cut off the current supply. The current drops down to a second predetermined reference value and on reaching it the driver stage is activated once more.
The state of the art has provided several approaches to the problem of sensing a current flowing through a load.
The current measurement has been usually obtained indirectly by measuring the voltage drop across a resistive sensor consisting of a resistor Rs having a very low value which is connected serially to a transistor pair in the bridge circuit performing the functions of so-called high-side drivers, and accordingly having their respective drain electrodes connected to the load.
However, this prior approach has some drawbacks as pointed out below.
Its accuracy is poor because said resistor Rs and the internal resistances of the integrated sensing circuit are liable to undergo thermal drifts which differ greatly from one another, which reflects adversely on the measurement accuracy.
Further, through the resistive sensor Rs, power is dissipated in an amount equal to the product of the voltage drop thereacross by the current IL flowing through the load. To restrain that dissipation, one might think of using very low resistance values, but this would result in the need to sense voltage values just as low, for subsequent comparison with reference values generated inside the circuit.
In recent times, it has been proposed of obtaining the measurement through the use of a pair of transistors connected together through their respective drain and gate electrodes. One transistor is incorporated to the bridge driver circuit for the inductive load, and the other transistor has its source electrode grounded via the resistive sensor formed of the resistor Rs.
If the second-mentioned transistor is provided with an area which is n times smaller than that of the first-mentioned transistor, then it becomes possible to have a current flow therethrough, and hence the resistor Rs, which is 0 times lower than the current flowing through the inductive load, whereby a definitely lower amount of power would be dissipated through the electric current sensor.
However, not even this prior attempt has been entirely successful, mainly because the very presence of the resistor Rs makes the values unequal of the gate-source and drain-source voltages at the previously mentioned transistors, which are therefore under different operating conditions. This results in the electric current ratii of the two transistors becoming dependent on such different operating conditions, rather than on the ratio of their areas, which again makes the measurement inaccurate.
In an effort to obviate such a problem, a circuit structure has been proposed as described, for instance, in Italian Patent Application No. 22732-A/86 by this same Applicant.
That structure comprises a pair of field-effect transistors connected together through their respective source electrodes. A first transistor is a power transistor and connected to the load, whereas the second transistor has its drain electrode connected to a resistive current sensor. In addition, a voltage regulator is connected to the drain electrodes of the first and second transistors, respectively, to maintain equal values of the drain-source voltage on both transistors.
The latter approach, while being beneficial and substantially achieving its objective, has a shortcoming in that it requires a purposely provided circuitry to drive the transistors in the pair to operate under the same conditions.